Gang Nail Trussed Rafter Manual

1SPECIAL DETAILS 57

Water Tank Location And SupportSupport For ServicesAccess To ServicesSite InfillWide Eaves SoffitsCantilevered Hip EndsGable LaddersLateral Support To Walls At Roof LevelExtraneous SupportFire PrecautionsPreventing Fire Spread Between Dwellings

BRACING 74

Bracing FunctionStability BracingWind Bracing

SPECIFICATION AND QUOTATION REQUIREMENTS 84

SpecificationInformation Required By The Trussed Rafter DesignerQuotationInformation Provided By The Trussed Rafter Designer

SITE PRACTICE 86

DeliverySite StorageHandlingErection ProcedureFixingSymmetryErection of Hip EndsTolerancesRemedial Work

94GLOSSARY

SECTION 9

SECTION 8

SECTION 7

SECTION 6

CONTENTS

Route Map 1

Introduction 3

TRUSS MECHANICS, MATERIALS AND RESPONSIBILITIES 4

Tension MembersCompression MembersTrussed RaftersCombined Stress IndexConnector PlatesTimberMoisture ContentsTreatment Of TimberLoad DurationBritish Standards And Codes Of PracticeDesign Responsibilities

ROOF AND TRUSS FORMATIONS 11

Roof ShapesTruss Shapes And SpansCommon Truss ModificationsRafter AlignmentFascia AlignmentManufacture And DeliveryHandling And Site Access

DESIGN LOADS 22

Dead LoadsImposed LoadsWind LoadsLoading Conditions

FORMING THE ROOFSCAPE 24

T Intersections And Valley InfillHip SystemsCranked Or Dogleg IntersectionScissor TrussesMultipart TrussesAttic Trussed Rafters

SUPPORT CONDITIONS 50

Eaves And Support DetailsTruss Fixing DetailsMultiple Trussed RaftersSupport Provided By Masonry

SECTION 5

SECTION 4

SECTION 3

SECTION 2

SECTION 1

ROUTE MAPTHE TRUSSED RAFTER MANUAL

3The Trussed Rafter Manual is a comprehensive reference guide to trussedrafter roof design, specification and construction. It is specifically designed tomeet the information needs of all the members of the project team, fromfeasibility planning, through the detail design stage, to erection andcompletion on site. Consequently, the Manual will be of use to a broad rangeof specifier groups, including architects, engineers, contractors, developersand also students.

Gang-Nail Systems Ltd, in conjunction with their UK and Eire network of fabricators, have beenat the forefront of trussed rafter technology for 40 years. Their ongoing involvement in thedevelopment of British Standards and Eurocodes is a testament to this fact. The Manual notonly draws upon this accumulated wealth of knowledge and experience, but also incorporateswork carried out by the Building Research Establishment, Trussed Rafter Association and BritishStandards Institution.

Gang-Nail's role within the construction industry is to support a network of timber trussed raftermanufacturers, by manufacturing and supplying punched metal plate connectors, anddeveloping the applications software necessary to design and supply prefabricated trussedrafters to the highest specification.

The extent to which primary product manufacture is combined with a sophisticated and highlydeveloped range of support services for client fabricator companies is seldom realised. Paralleldevelopments in both the design and client services offered by Gang-Nail Fabricators has meantthe combined resources of the System Owner and Fabricator network can respond to the mostdemanding requirements from the roofing sector.

Since the establishment of Gang-Nail Systems Ltd the range and complexity of projectsdesigned, supplied and erected under the System by fabricator companies has progressedrapidly. Housing applications are well known, with an estimated 95% of new build housingutilising prefabricated trusses. Less well known is the extent to which trussed rafter roofstructures are now part of the non-domestic building scene. The aesthetic and architecturalappeal of pitched roofs has spawned a rich variety of commercial and industrial applications,including offices, shopping centre, superstores, hospitals, schools, hotels and light industrialbuildings. An estimated 45% of the annual output from the trussed rafter industry is presentlydirected towards these types of project.

Gang-Nail's preparation of The Trussed Rafter Manual has taken this continuing trend intoaccount by approaching its subject not only in the context of domestic dwelling requirements,but also in terms of what can be achieved on larger scale developments.

It is important to recognise that The Trussed Rafter Manual sets out to inform and guide theproject team and not to replace the Structural Engineer. It is essential that the Building Designer,who is assumed to have ultimate responsibility for all aspects of the project, considers andapproves the information given in relation to the specific project under consideration. If used inthis way the aim of the Manual will have been fulfilled, and its rightful position on the desk of all relevant parties will be assured.

Comment

Section 2 gives some guidance on what can be achieved.

When the required guidance is extensive, the fabricator may charge afee. In this situation, the Client may consider nominating a supplier towork with the building team. This reduces the initial costs and allowsproblems to be solved at the pre-tender stage, resulting in better controlat the construction stage, which often equates to a cost saving.

Avoid itemising trusses in a Bill of Quantities (see Section 10).

On larger jobs, give an indication of how the contract will be phased.

Provide the information listed in Section 10, thereby ensuring that thefabricator has enough detail to produce an accurate quotation.

In addition to designing and manufacturing the trussed rafters, somefabricators will undertake the design and detailing of the completestructural roof including the stability bracing.

Trussed rafters are designed and manufactured to suit each contract.Adequate time must therefore be allowed between placing the orderand requiring the trusses on site. As a guide, it is recommended thatorders are placed at least 4 weeks prior to required delivery. On largerjobs, this period may extend to several months.

In addition to designing and manufacturing the trussed rafters, somefabricators will undertake the erection of the roof.

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INTRODUCTION

5Trussed RaftersTo support an 8m span roof with beams at 600mm centres requires 100mm x 350mm timber members. The equivalent trussed rafter, assuming a pitch of 30, would use much smaller member sizes and only a quarter of the timber (Figure 1.04). How is thispossible?

Consider two rafter members - AB and BC - in contact at B and restrained from moving at A and C (Figure 1.05). It is possible to suspend a weight W from nodeB, placing AB and BC in compression; the rope is in tension.

Relating this to a truss (Figure 1.06) points A and C are prevented from spreading by the bottom chord AC which is in tension and ties the rafters together. For this reason, bottom chord AC is often called the ceiling tie.

FIGURE 1.06 TRUSS ACTION

An 8m long, 35mm x 97mm ceiling tie would deflect and so it is supported at F and G by members BF and BG which hang from node B and are in tension. The length of members AB and BC would also result in excessive deflection so they are propped at D and E by members DF and EG which are in compression. In this way it is possible to increase the span range for a given timber size by simply adding more tension and compression webs, as shown in Figure 1.07, assuming chord members 35mm x 97mm throughout.

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force is not only dependent on raw material strength andcross-sectional area, but also member length and minimumbreadth. A simple experiment will demonstrate this. Hold a 300mm scale rule vertically on the desk and pushdown on it. At a relatively small load it will buckle. Repeatthe experiment with a 150mm long scale rule, it will take amuch greater force before it buckles. If we were to takea 10mm length of the scale rule and subject that tocompression, it would eventually crush at a relativelyenormous load and be said to fail in compression asopposed to buckling. This experiment demonstratesthe principle that as member length increases so loadcarrying capacity decreases. Similar experimentskeeping length and cross-sectional area constant wouldshow that load capacity decreases as the cross-sectionchanges from a square to a long rectangle (Figure 1. 02).

Tension MembersA member subject to a tension force is being pulled orstretched; it is said to be in tension. Common examplesof a tension member are a car tow rope and the ropesupporting a child's swing. The ability of a member torestrain tension forces depends on the raw materialstrength of the member and its cross-sectional area.Illustrated in Figure 1. 01 is the force a 50mm x 50mmmember can withstand and a 100mm x 50mmmember, which being twice the area carries twice theforce.

Compression MembersA member subject to a compression force is beingpushed or compressed; it is said to be in compression.Common examples of compression members arecolumns or the leg of a table. Unlike the tension memberthe ability of a member to resist an applied compression

Combining these two findings indicates that loadcapacity is dependent upon the slenderness ratio; that is

Member LengthSlenderness Ratio = ___________________________

Minimum Cross-sectional Dimension

If we rigidly supported the 25mm x 100mm memberlaterally at mid-point in the 25mm direction, the lengthover which it can buckle is halved and hence itsslenderness ratio would be equal to that for the 50mmx 50mm section and the allowable load wouldincrease from 4.8kN to 12.6kN (Figure 1.03). Thisexplains the importance of lateral bracing anddemonstrates why, when requested by the TrussedRafter Designer, lateral braces must be correctlyinstalled.

The modern trussed rafter roof has evolved in form overthe past forty years. The speed with which it can bemanufactured and erected, along with its efficiency ofmaterial use has meant that it is now used in the vastmajority of domestic roofs and increasingly forcommercial construction projects. Before providingdetailed guidance on all aspects of trussed rafter roofdesign and construction, a brief description is providedin this section of the basic structural mechanics oftrusses, the materials which are used, and theresponsibilities of the various parties involved in theirdesign. Roof trusses are primarily made up of tensionand compression members; so what are tension andcompression members and how do they behave?

SECTION 1

TRUSS MECHANICS, MATERIALS AND RESPONSIBILITIES

SECTION 1


7GN80XAn 18 gauge (1.2mm thick) plate manufactured from highstrength steel and normally used for splices.

GN14GN14 is a 14 gauge connector plate manufacturedfrom steel nominally 2mm thick (Figure 1.08). GN14plates are predominantly used for special applicationswhere very large joint forces occur. The length of thenail is 20mm and hence the Agrement Certificateprohibits their use in timber less than 44mm thick.Available widths are 66mm, 76mm, 114mm, 133mm,152mm, 190mm and 228mm in lengths from 100mmup to 1220mm.

Field Splice PlatesA plate with one half consisting of Gang Nails and the otherholes to receive square twist nails. Used to fix a 2 part trusstogether on site e.g. top hat attic truss.

This example illustrates that the members in a truss are subjected to axial forces (i.e. tension or compression). Since members can resist axial forces more easily than they can bending forces, smaller timber sections canbe used in a truss than an equivalent beam.

Combined Stress IndexIn reality trussed rafter chords receive load along their entire length and they are therefore subject to bending forces. The same basic theory, however, still applies, except that instead of all the member strength being used for axial forces (tension or compression), some of it is required to resist the bending forces. The design calculation for the trussed rafter will state the Combined Stress Index (C.S.I.) for each member,which should not exceed 1.0. A value of 1.0 says themember is stressed to the maximum permitted value:in tension, compression, bending, or a combination ofthese. Some examples are as follows:Rafter C.S.I. = 0.81 81% of the strength of the

rafter is being used to resistcombined bending and compression forces

Ceiling Tie C.S.I. = 0.49 49% of the strength of the tie is being used to resist combined bending and tension forces

Tension Web C.S.I. = 0.73 73% of the strength of the web is being used to resist tension forces

Connector Plates Gang-Nail punched metal plate fasteners aremanufactured from galvanised mild steel. Rows of integral nails are pressed out to project at right angles to one face of the plate (Figure 1.08). The slots so formed define the length direction of the fastener. One nail is formed from each slot, with alternative rows of nails facing in opposite directions. The nails are formed with a slightly dished cross-section.

The use of Gang-Nail connector plates is covered by British Board of Agrement Certificates. The fasteners are stamped with the identification mark: GN20, GN14 or GN80X.

GN20GN20 plates are available in widths of 50mm, 63mm, 76mm, 101mm, 127mm and 152mm and lengths from 71mm up to 1220mm, although the maximum length normally used is 401 mm.

The fasteners are manufactured from carbon steel of nominal 1mm finished thickness and protected against corrosion by hot-dip galvanising with a minimum zinc coating weight of 275 g/m2 The steel specification is in accordance with BS EN 10326.

Some typical joints with their load carrying capacities are given in the diagrams shown as Figure 1.09.

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GN20

GN14

Field Splice Plate

GN80X

FIGURE 1.08 THE GANG-NAIL CONNECTOR PLATE RANGE

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9Where cross cutting is carried out after treatment, all sawn ends should be given the appropriate treatment required by the relevant preservative or treatment specification,before assembly.

Organic solvent type preservatives lend themselves tomodern industrialised techniques for the fabrication oftrussed rafters, since punched metal plate fasteners may be pressed into the timber shortly after treatment.

Copper/Chrome/Arsenic (CCA) preservative should not beused because of the possible risk of corrosion of punchedmetal plate fasteners and nails.

Galvanised punched metal plate fasteners and nails shouldnot be used in timber which has been treated with a flameretardant.

Load DurationThe grade stresses and joint strengths given in BS5268:Part 2 are for long-term loading. Timber can howeversustain greater loads for a period of a few minutes than for a period of several years and BS5268: Part 2 reflects thisfact in quoting load duration factors by which the gradestresses can be modified. These factors are given in Table1.02 and apply to all strength properties but not moduli ofelasticity or shear moduli (see also Section 3).

TABLE 1.02: MODIFICATION FACTOR K3 FOR DURATIONOF LOADING

Duration of loading Value of K3Long term 1.00Medium term (e.g. dead + snow,dead + temporary imposed) 1.25Short term (e.g. dead + imposed + wind, 1.50dead + imposed + snow + wind)Very short term(e.g. dead + imposed + wind) 1.75

British Standards and Codes of PracticeThere is one major British Standard applicable to trussedrafters, namely BS5268:Part 3:A brief outline of this andrelevant supplementary documents follows.

BS5268: Part 3: Code of Practice for trussed rafter roofs -regulates the materials and design methods used. It encompasses information on the handling, storage andsite erection of trusses. Comprehensive guidance is alsogiven on stability bracing and overall roof bracing.

BS5268: Part 2: Code of Practice for permissible stressdesign, materials and workmanship - provides basic stressdata for the structural timbers to be used in manfacturingthe trusses.

BS EN 14250: Product requirements for trussed rafters -gives all the rules required to manufacture trusses withpunched metal plates.

BS EN 519: Softwood grades for structural use-sets out thebasic rules for stress grading timber, including an explanationof and limits for visual defects.

BS6399: Part 1: Code of Practice for dead and imposed loads- stipulates the intensity of load that structures should bedesigned for.

BS6399: Part 3: Code of Practice for imposed roof loads -colloquially referred to as the snow code, this document givesdetailed guidance on the snow loads to be used in the designof a roof, in particular drift loads.

BS6399: Part 2: Wind Loads - provides data to enable anassessment of the wind loads on a structure.

BS648: Schedule of weights of building materials - givestypical recognised weights for materials.

The manufacture and design of trussed rafters is reliant uponcomputer software. The programs are complex and revisionstake time to introduce. It is not always feasible, therefore, forrevisions to be implemented on the day an amendment to aCode is issued.

Design ResponsibilitiesTo avoid misunderstanding and confusion, it is essential that incontracts involving trussed rafters both the supplier andcustomer clearly understand the legal responsibilities of eachparty.

On every project, no matter how small, a person must be giventhe overall responsibility of Building Designer and clearlydefined as such. As this person requires detailed knowledge ofthe design assumptions for the entire building it is generallyimpractical for the Trussed Rafter Designer or Roof Designer to assume this role. To assist in the clear understanding of theabove functions, the definitions of the various parties involvedis stated.

The Building Designer may be the owner of the building, hisappointed architect, a structural engineer appointed by theowner or his architect or, in the case of small buildings, theactual builder. The Building Designer should ensure that thedesign of the roof as a whole, and its connection to, andcompatibility with, the supporting structure and adjacentelements of the building are satisfactory with regard to theoverall stability of the complete structure. The Building Designer should note also any stability requirements specifiedby the Trussed Rafter Designer and should ensure that theserequirements are incorporated in the complete structure.

The design of the roof should be checked by the BuildingDesigner to determine if an adequate margin of safety existsagainst uplift due to wind forces and, when required, adequateholding down fixings are specifiedfor both the trussed raftersand the wall plates or bearings. The Building Designer isresponsible for detailing the bracing necessary to provide therestraintsrequired by the Trussed Rafter Designer.

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(2) In unseasoned timber, water is held partly in the cellwalls and partly as free moisture within the cell cavities. As the timber dries, the free moisture is evaporated before the cell walls lose their water. Fibre saturation is defined as the condition when all the free water has been removed but the cells are still saturated.

Below the fibre saturation point, changes in moisturecontent are accompanied by shrinkage of the wood and anincrease in most strength properties. To account for theshrinkage, timber sizes are normally related to a moisturecontent of 20%.

The relationship between moisture content and strengthdiffers for each property but, as an indication, Figure 1.10relates moisture content to compression strength.

Below the fibre saturation point, strength increasessignificantly with reducing moisture content. Havingadopted 'dry stresses' for the design, the significance ofensuring the moisture content does not exceed 18% will be apparent.

Treatment of TimberThe risk of rot or insect attack in the timber of wellventilated pitched roofs is regarded by BS5268:Part 5as low, except in those areas specified in the BuildingRegulations as subject to infestation by the houselonghorn beetle (Hylotrupes bajulus L). The preservativetreatment of trussed rafters, other than in thesespecified areas, may be regarded as unnecessary exceptas an insurance against the cost of possible repairs.Where preservative treatment is required, it shouldsatisfy the requirements of the Building Regulations.The type of preservative used should neither increasethe risk of corrosion of punched metal plate fastenersor nails. The recommendations of BS5268:Part 5 in thisrespect should be followed.

TimberAll timber used in the manufacture of trussed rafters mustbe stress graded. Some of the common species are givenin Table 1.01, taken from BS5268:Part 3.

TABLE 1.01: SPECIES OF TIMBER

Standard name Origin

WhitewoodRedwood Europe

Hem-firDouglas fir-larchSpruce-pine-fir Canada

Southern pineHem-firDouglas fir-larch USA

Scots pineCorsican pine Britain

European redwood and whitewood are imported as amixed parcel of timber, the majority of which is whitewood.These timbers form the bulk of all trussed raftersmanufactured and it is important that specifiers do not try to separate them.

The normal grades of European redwood/whitewoodemployed are TR26 and C16.

The most common widths of timber are 35mm or 47mm,and depths range from 72mm to 197mm for 35mmthickness and 72mm to 244mm for 47mm thick timbers.For webs, a depth of 60mm is permissible.

The timber is prepared on all faces, often to what the timbertrade refers to as hit and miss planing. A fully planed finish,as with joinery timbers, may not be achieved, but themajority of the sawing marks are moved.

Moisture ContentsTrussed rafters are assumed to satisfy the 'Dry Exposure'conditions defined by BS5268:Part 2, whereby the moisturecontent of the timber must not exceed 18% for anysignificant period. BSEN 14250 does permit the moisturecontent to be 22% at the time of manufacture, recognisingthat the timber will dry tobelow the 18% value during theconstruction phase and before the majority of the designload is applied. Typical values recorded in occupiedbuildings range from 10% to 16%

The control of the moisture content of timber is importantsince it influences the properties of the timber. For example:

(1) Below a moisture content of 25% wood is less prone todecay and may be considered immune below 20%.

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Following an introduction to basic roof and truss shapes,common modifications are discussed along with factorswhich influence the choice of truss. Detailed guidance onforming the roof is given in Section 4, in particular hipsystems and roof intersections.

Roof ShapesIt is now fully accepted that trussed rafters provide aneconomic structural roof solution. With more emphasisbeing place on the appearance of buildings they also allowthe architect virtually free expression when designing theroofscape.

Domestic Roofs - Roofs for housing and similar typebuildings may be a variety of shapes. The shapes are

dictated primarily by the floor plan, followed by architecturaland engineering considerations. Illustrated in Figure 2.01are some of the more common basic shapes which canoccur in isolation or in combination with other shapes.

Commercial and Industrial Roofs - In principle, thevariety of shapes and layouts depicted for domestic typeroofing apply also to commercial and industrial buildings.Spans may, however, be larger and loads considerablyhigher, making it necessary to treat eac project on itsmerits.

Ideally the Gang-Nail fabricator should be consulted atthe feasibility stage.

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The Trussed Rafter Designer designs and details theindividual trussed rafters, clearly stating their size, loadingand support conditions, stating the points of lateralrestraint required to prevent buckling of compression andrafter members and, where necessary, internal members.The Trussed Rafter Designer should receive informationfrom the client or his agent as listed in Clause 11.1 ofBS5268:Part 3 and provide information in return as listed inClause 11.2 (Further guidance is given in Section 8). The Trussed Rafter Designer is usually the truss fabricatorand his supporting System Owner.

The Roof Designer may be appointed by the BuildingDesigner to carry out that part of the Building Designer'sduties which relate to the roof structure. The Roof Designerwould normally liaise with the Trussed Rafter Designer toensure that all structural aspects of the roof are considered.He would also require information from the BuildingDesigner with regard to wind loading, location and size of shear or buttressing walls and deflection criteria.

It is recommended that the above terms of BuildingDesigner, Trussed Rafter Designer and Roof Designer are used in contractual documents for the sake of clarity ofmeaning.

SECTION 1


SECTION 2

ROOF AND TRUSS FORMATIONS

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Common Truss ModificationsIncreasingly, on domestic and on most commercial/industrial projects, standard trusses must be modifiedto suit architectural and structural requirements.Stubbed, cantilevered and extended chord trusses areby far the most common modifications required andthese are discussed in detail.

Stubbed TrussesWhere a full profile truss is truncated, as in Figures 2.05and 2.06, it is referred to as a stubbed truss. Sincestubbed trusses usually occur with full profile trusses,they are normally derived from the geometry of theprofile truss (e.g. Truss B in Figure 2.05). This is forseveral practical and economic reasons:

(1) Helps to maintain rafter alignment (see later).

(2) Minimises production 'downtime' in resetting thejig, since only a few adjustments are required.

(3) The majority of timber components are common totrusses A and B, which reduces cutting time.

(4) Alignment of webs helps detailing and installationof the stability bracing and services.

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Truss Shapes and SpansThe selection of truss shape is dependent on span,loadings, rafter alignment (discussed later) and timbersize limitations. It is therefore best left to the fabricatorto decide on the profile to be used. As an indication,however, the most common truss profiles are shown inFigures 2.02 and 2.03. The normal economic span isshown, although greater spans can be achieved.

The names for monopitch trussed rafters are derivedfrom the number of bays the top and bottom chordsare divided into. For example, a 2 on 1 (or 2/1) will have2 top chord bays and 1 bottom chord bay.

The required dimensions and reference points forduopitch and monopitch trussed rafters are shown inFigure 2.04. It is worth emphasising the followingpoints:

(1) The outside face of the wall plate is oftenlocated at the Setting Out Point (S.O.P.) and consequentlyspan overall supports equals span overall S.O.P.'s. The two spans should however be thought of as beingseparate since in all modified trusses they will not be equal.

(2) The overhang and soffit width are not the samedimension. Both are measured to the back of thefascia, but the former is from the S.O.P. and the latterthe outside face of the brickwork. For trussed rafters,the required overhang dimension and end cut shouldbe given.

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The structural treatment of cantilevered trusses varieswith increasing cantilever distance:

(1) Standard HeelBS5268:Part 3: permits a small cantilever on normalheel joints without further modification, as shown inFigure 2.09.

(2) Modified HeelIn some instances, cantilevers greater than (1) can beaccommodated by modifying the heel joint, as shownin Figure 2.10. Limitations depend on loads and timbersizes, but the support will always be local to the heeljoint.

(3) Cantilever WebWhere the support occurs outside of (1) and (2), acantilever web is added to strengthen the bottom chord,as shown in Figure 2.08. The maximum cantilever distancepermitted is normally limited to the lesser of a quarter ofthe setting out points span, or the first internal node point.

In some instances a cantilever causes the outer bottomchord bay to be in compression and a lateral brace maybe required, as shown in Figure 2.11. The TrussedRafter Designer will advise when this is to be provided.

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To be specific, stubbed trusses should be referred to as,for example, stubbed fink or stubbed 2 on 2 monopitchtrusses, depending on the profile they are derived from.

In the rare cases of stubbed trusses occurring in isolation, it may be advantageous to produce a special design withthe nodes adjusted to balance the chordbay lengths.

There are no limitations on the amount trusses can bestubbed but, to prevent large uplift forces occurring, anapproximate 'rule of thumb' is to ensure the final spanis not less than the height of the truss.

When specifying stubbed trusses, the dimensions givenin Figure 2.07 should be stated. The span over settingout points allows a check to be made that the otherdimensions are correct and no misunderstandings haveoccurred.

A range of support and end conditions are described inSection 5.

Cantilevered TrussesA cantilevered truss occurs where the main body of thetruss, not just the rafter overhang, projects outside thesupport, as shown in Figure 2.08. When referring tocantilevered trusses, the profile name should be used.For example, Figure 2.08 shows a cantilevered fan truss.All the standard profiles can be cantilevered at one orboth ends.

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Hip end and roof intersections present specificproblems and should be discussed with a Gang-Nailfabricator.

Structurally, the extended legs support the weight ofthe roof and resist the large bending forces imposedupon them. Additionally, the horizontal deflectionsthat occur on raised tie trusses must be contained to afigure the supporting structure can accept. (For moststructures it is recognised that 6mm can be tolerated ateach support.) To achieve this, the extended legs mustbe strengthened using one of the following threemethods:

(1) Increased Depth of ChordThe simplest solution is to increase the depth of theextended chord member. Figure 2.14 compares astandard fink and raised tie trussed rafter of equalspan. It can be seen that the top chord hasincreased from 72mm to 169mm in depth.

(2) Add Scabs

Preferably the scabs should be nailed to the truss bythe fabricator, since they are a vital part of the structureand represent over 50% of the bending strength of theextended leg. Where they are to be site fixed, a nailingpattern must be obtained from the fabricator and strictlyadhered to.

For some designs, bolts may be specified instead ofnails. Bolts should be treated against corrosion andsupplied complete with two plate washers to preventthe nut and bolt head from being drawn into the timber.

(3) Superchord

The extended chord can be reinforced by nailing orbolting additional members (scabs) to it, as shownin Figure 2.15. Repeating the above example resultsin a 145mm deep top chord with one scab or120mm top chord with two scabs.

The term superchord describes deep chord structuralmembers that are formed by stitching two smaller timbersections together using Gang-Nail connector plates, asillustrated in Figure 2.16. Whereas for solid timber the maximum sections available are 35mm x 197mmand 47mm x 244mm, superchords 35mm x 314mm and 47mm x 388mm can be produced from stocktimber sizes.

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When specifying cantilevered trusses, the dimensionsgiven in Figure 2.08 should be provided. Sincetraditionally the cantilever distance has been measuredto either the centreline or the outside face of the wallplate, all the dimensions shown should be provided toensure no misunderstanding occurs.

Extended Chord TrussesExtended chord trusses occur in two principal forms,either extended top chords or extended bottom chords,as shown in Figure 2.12. In both cases, the supportoccurs on the extended member. They arepredominantly used in conjunction with the fink or queenpost truss but, with the exception of scissor and flattrusses, can be applied to other truss families.

Since the bottom chord, or tie member, does not occurat wall plate level but is raised, extended top chords arecommonly referred to as raised tie trusses. The lowereaves height produces a cottage effect, allowing thenew structure to blend in with period properties and,consequently, is attractive to some planning authorities.Other applications are to allow increased internal roomheight or as a design feature.

The extended bottom chord is normally used overdormer windows in raised tie roofs, as shown in Figure2.13. It is also useful where the distance between thesupport wall varies. Designed for the maximum case,the extended leg can be cut back on site to take up thereducing span.

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19

The fabricator if provided with the correct information,would normally make this adjustment automatically. Tocompensate for the increased chord depth, truss A wouldbe revised to a 2 on 2 monopitch profile, thereby saving aweb member. The alternative solution is to cantilever trussA a sufficient distance to 'line-up' the rafter, but practicalproblems are such that this approach is rarely adopted.

Rafter AlignmentWhere more than one design of trussed rafter is employedon a roof, the rafters for the various sections must align.This is referred to as line-up' and is illustrated in Figure2.21. There are two ways to align rafters. The preferredsolution, and by far the simplest, is to make the top chorddepth on trusses A, B and C all the same.

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It is particularly suited to extended chord trussed rafters(Figure 2.17) offering not only significant economies inmanufacture and delivery but also providing the architectwith greater freedom in design.

As an alternative to reinforcing the extended legs, it may bepreferable to use cross wall construction, as shown in Figure2.18. By supporting the body of the truss on the beams, theload is relieved from the extended leg allowing smaller timbersections to be used and longer rafter extensions.

To specify extended chord trusses provide the dimensions, as illustrated in Figure 2.19. Try to avoid extended legsgreater than 0.9m, unless alternative methods of supportcan be provided.

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Handling and Site AccessTrussed rafters can be large, flexible, heavy units and it is important to consider the handling of them andsite access at an early stage. Where cranes cannot beused, unit weight is important. Illustrated are the weights of three typical trussed rafters.

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Fascia AlignmentFascia alignment on asymmetric roofs requires separateconsideration. Where roofs of different pitch intersect,thefascia board is usually aligned. This can be achieved in twoways, as shown in Figure 2.22. It must be made clear to the fabricator which detail is required.

Timber SectionsTo ensure the trussed rafter has sufficient robustness towithstand reasonable site handling, BS5268:Part3 requiresthat it should be a minimum of 35mm thick for spans up to 11m and 47mm thick for a 16m span.

Within and above this range of spans the minimumthickness should be obtained by linear interpolation orextrapolation. The trussed rafter may be manufacturedto the required thickness as one unit or consist of twoor more rafters, each not less than 35mm thick,permanently fastened together at the fabricator's works.The maximum bay and web lengths are also limited tothose given in Tables 2.01 and 2.02.

Manufacture and DeliveryTo suit manufacturing and delivery requirements, thenormal size range for trussed rafters in one piece is forspans up to 16 metres and heights up to 4 metres.Absolute limits depend upon available delivery routes andmanufacturing equipment, and spans up to 20 metres andheights in excess of 5 metres have been achieved.

For trussed rafters manufactured and delivered in two ormore parts, only design parameters limit what can beachieved as illustrated by the following examples of pastjobs.

1.9 2.5 3.3 3.3

2.6 3.93.6

4.1

4.35.05.3

3.55.2

SECTION 2


SECTION 2


23

(iii) Reduced surface area and hence lower weight ofroof structure.

The need to consider uplift was well illustrated in 'Galedamage to buildings in the UK - an illustrated review' by P.S.J. Buller, published in 1986 by the BuildingResearch Establishment (BRE).

To enable a design check to be carried out, the specifiermust give either specific wind data or the grid coordinatesof the site.

It is normal practice when designing a trussed rafter tooverstate the dead loads to allow for uncertainty, ultimatelyproducing a safe structure. When carrying out a check onwind uplift this could lead to an unsafe structure, since it is thedead load that resists the uplift forces. The MINIMUMexpected dead loads should therefore also be stated.

Attic Trussed Rafters-Special LoadingConsiderationsAttic trussed rafters support extra dead and imposed loads.

Dead Loads - Floor boarding will be required on thebottom chord and plasterboard to the walls and ceiling.

Imposed Loads - BS6399:Part 1 requires an imposed load of 1.5kN/m2 over the floor area for domestic buildings.Greater values will be required for other types of use anddetails should be provided by the specifier.

Loading ConditionsTimber members can, as explained in Section 1, sustainvery much greater load for a period of a few minutesthan for a period of several years. Where appropriate,a check is made on three periods of loading: long,medium, and short-term. These loading conditionscomply with BS5268:Part 3 and are made up as follows:

Long-TermLong-term loads comprise dead loads on the top chord anddead plus permanent imposed load on the bottom chord. The tank load, if applicable, is placed in the bay where thetank is to be situated.

Medium-TermLong-term.loads, as above, plus the imposed top chord(snow) loading.

Short-TermMedium-term loads, as above, plus the addition of a 900Nman load modified in accordance with BS5268:Part3. Theman load should be placed in any position so as to producethe maximum stress and reactions in the members.

Occasionally it is also necessary to consider wind gustforces that are very short-term. This load case thereforeconsists of long-term loads, as above, plus wind loads.

TABLE 3.01: SUMMARY OF LOADS AND LOAD CASES

Rafter Loads

Dead

For concrete interlockingtiles 685N/m UDL(measured along theslope) or as specified

Imposed

Snow Load as BS6399:Part 3 (except drift loads)

Drift Loads

OR

900N man load

Wind

Wind calculatedaccording to BS6399:Part 2

Ceiling Tie Loads

Dead

250N/m UDL

PLUS

2 x 450N concentratedloads for water tank oractual load if greater

PLUS

Service/fittings loads

Imposed

250N/m UDL

900N man load reducedwhere appropriate to675N

Location

Full length

Full length

Full length

Centre of Bay

Full length

Location

Full length

At 2 nodesnearest watertank

As appropriate

Full length

Centre oreither end ofany bay

Duration

Long-term

Medium-term

*

Short-term

Very short term

Duration

Long-term

Long-term

Long-term

Long-term

Short-term

* Drift loads are included in a special category ofloading, termed Accidental. Accidental loads aresubject to reduced factors of safety.

22

Trussed rafters are precisely engineered structuralcomponents, the design of which is dependent on the loadsadopted. The following serves to assist the specifier inunderstanding and evaluating design loads.

Dead LoadsDead loads are the loads that make up the permanentstructure. They include:

Roof FinishesRoof finishes vary in weight from light aluminium sheetingswhich are less than 100N/m2, to natural slate tiles, such asYork stone, which can exceed 2500N/m2. Manufacturers ofroof tiles give 'laid weights'. BS5268:Part 3 suggests a valueof 575N/m2 for common concrete interlocking tiles. An additional allowance of 110N/m2 for felt, battens and therafter is usually adequate. Thus, total dead load for concreteinterlocking tiles is:

Laid Weight = 575 N/m2

Felt, Battens, Rafter = 110 N/m2

Total 685 N/m2

Ceiling FinishesA load of 250N/m2 will take account of 12mmplasterboard, skim coat, noggings, insulation and selfweightof the ceiling joist. Where suspended ceilings are proposed,laid weights should be obtained from the ceiling manufacturer.

Water TanksBS5268:Part 3 requires an allowance to be made for awater tank unless there is specific information to thecontrary. To allow for 230 litre or 300 litre net capacitytanks, supported as described in. Section 6, a load of900N per truss is applied as two node point loads of 450N.

In non-domestic properties where larger tanks are required,the location, size and weight of the tank must be given. For exceptionally large tanks, support independent of thetrusses may be preferable.

ServicesExcept for special cases, such as communal heating andwater systems, the loads from services on domesticstructures can be ignored. On other types of building, such ashospitals, schools and offices, service loads can be significantand should be assessed.

Service layouts will be modified up to and even afterinstallation. It is wrong therefore to specify discrete loads forwhich individual trusses must be designed, especially as it isdifficult to ensure that the particular truss will be erected in thespecified position on site. The only practical solution is toagree with the services engineer, at the outset, a uniformlydistributed load to be used either over the whole roof or overspecific areas. Typically a value of 250N/m2 to 500N/m2 isused. This involves a small element of overdesigning but thecost is fully recovered in the flexibility it allows the

services contractor and by project time not being wasteddesigning remedial works to allow plant repositioning.Exceptions to a uniformly distributed load are main elements ofplant, such as air handling units. For these, specific locations,weights and sizes must be given.

If a boarded walkway is to be provided in the roof void,allowing easier access for maintenance staff, the extentand detail of the walkway must be given. See Section 6 forinformation on support requirements.

FittingsWhere fittings are to be suspended from or supported by thetrussed rafters, a description and unit weight is required.Typical examples are folding partitions, chandeliers, cupolas or clock towers.

Fire BarriersTo provide a horizontal fire barrier, additional or thickerlayers of plasterboard may have to be nailed to the ceiling.Vertical barriers can be achieved by nailing two layers ofplasterboard to the face of a truss. In both cases the fabricatormust be informed so that the extra loads are included in the design.

Imposed LoadsImposed loads are determined according to the intendedoccupancy or use of a building, including the weight ofmoveable partitions, furniture, people, stored materials andsnow on the roof.

Snow LoadsThe load to be applied is detailed in BS6399:Part 3. It depends on the geographical location and altitude of thesite, together with the roof geometry. This information must be provided by the specifier.

Bottom Chord Imposed LoadOn the majority of roof structures a light storage loadof 250N/m2 is allowed over the entire roof area. Thespecifier must inform the fabricator if a greater value isrequired. In addition, a 900N allowance for a man loadmust be considered, placed to give the maximum stresses.Where trusses are placed at maximum centres of 600mm and a plasterboard ceiling is applied, BS5268:Part 3 allows25% of this load to be redistributed onto the adjacent trusses.In this event the man load allowance on any one trussbecomes 675N.

Other Imposed LoadsDetails of any item, from climbing ropes in schools to bath liftsin old peoples' homes, must be provided.

Wind LoadsPrimarily, the effect of wind load on a roof is uplift. This isparticularly true the shallower the pitch, for three reasons:(i) Higher uplift forces occur.(ii) Lighter forms of construction are used.

SECTION 3

DESIGN LOADSSECTION 3

DESIGN LOADS

25

Main roof truss supported intruss shoe

24

This section builds on the introduction to trussed raftersgiven in Section 2, 'Roof and Truss Formations' andprovides more detailed guidance on forming theroofscape. Features, such as hips and roof intersections,are described along with trusses that require specialconsideration, such as the scissor and attic families.

T Intersections and Valley InfillWhere two roofs intersect at 90, a T intersection isformed. The oncoming ridge can be below, equal to, orabove the main ridge, and spans and roof pitches canvary. (Figure 4.01).

Case 1 is considered in Figure 4.02, but the principlesremain the same for Cases 2 and 3. The intersection isformed by the use of diminishing valley frames,collectively referred to as a valley set.

The valley frames transfer the rafter loads down ontothe underlying trusses in a uniform manner. To achievethis they require vertical webs at approximately1200mm centres and must be erected in firm contactwith each rafter they cross. Since the tile battens areomitted in the overlay roof area, supplementarymembers must be provided to laterally restrain therafters of the supporting trusses. Typically, tile battensare nailed to the underside of the truss top chord,extending 1200mm beyond the valley line.

SECTION 4

FORMING THE ROOFSCAPESECTION 4

FORMING THE ROOFSCAPE

27

(9) Maximum economy will be achieved by allowingthe fabricator to select the framing method thatbest suits his manufacturing process. With the exception ofthe site infill hip end, hip systems are all based aroundgirder and intermediate trusses of the same profile usingflying rafters. This is fully described for the standard centreship (Figures 4.05 and 4.06). Brief details are also given forother hip systems.

Standard Hip EndThere are five alternative methods of framing thestandard hip end:

(1) Standard Centres Hip-most common up to 11m;girder position fixed.

(2) Standard Set Back Hip-similar to (1) with girderposition flexible.

(3) Girder Based Hip - alternative to (1).

(4) Site Infill Hip - for small hips to 6m span.

(5) Two Stage Hip for large hips in excess of 11 m span.

The alternative to the 'flying rafter' method of constructinghips is the step-down hip. This is shown in Figure 4.04.Due to the increase in different truss profiles required, it isexpensive to produce, time consuming to design, difficult to brace and therefore rarely if ever used.

Mono truss to be supported on hipgirder truss D bottom chord bytruss hanger and fixed to topchord as Figure 4.05

26

Where there is no load bearing wall through theintersection, a girder truss will be required to carry theroof trusses over this opening, as shown in Figure 4.02.Due to the heavy loads being carried by these girders,a larger than normal bearing is often required. It isrecommended that consideration be given to thedesirability of using a concrete padstone for the girdersupport. Gang-Nail fabricators can supply informationon minimum bearing areas and girder support loads forindividual projects.

Valley frames are the most economic and structurallysound solution to T intersections, ensuring the load istransferred to the supporting trusses uniformly. Wherethe area is to be formed using site framing, it mustcontain a horizontal tie member and vertical postssimilar to those used in the valley frame. Nailed jointsare less efficient and require greater end/edge distancesthan Gang-Nail connector plates and consequently a50% increase in timber depth will be required.

Hip SystemsOther than the basic gable end, the hipped roof is themost common feature being incorporated into roofsand one of the most attractive. Figure 4.03 illustratesthe hip family.

General points to note are:(1) A hip system is the collective name for a group oftrussed rafters that form the hip.

(2) A hip end is a complex three dimensionalframework which, for simplicity, is treated as a twodimensional problem. In design, the hip board issized to satisfy an ultimate load criteria for safetyreasons. In practice, it carries negligible load.

(3) Although hips above 12m span are common andspans of 20m have been achieved, special detailsmay be required depending on roof pitch and thelocation of internal walls and consequently a Gang-Nail fabricator should be consulted at the planning stage.

(4) Generally the minimum preferred pitch is 22.5,which allows adequate depth for the girderssupporting the hip.

(5) Where the pitch on the end is different to that onthe sides, the specifier has two options: either thesteeper pitch truss must be cantilevered or the soffitwidth varied to maintain the eaves line (see Section2, Figure 2.22).

(6) All normal roof finishes can be used andmodifications, such as stubbed ends and smallcantilevers, can be incorporated.

(7) Through discussion with the supplier, hip girderscan be positioned at the design stage to avoidchimneys or to prevent large reactions occurringover windows, etc.

(8) In some instances, pre-made components can beprovided to simplify and speed up the constructionof the infill area.

SECTION 4


SECTION 4


29

The hip board is notched over the hip girder to provide asupport and taken to the apex of the hip, where it issupported on a ledger fixed to the last full profile truss.

The corner areas of the hip are completed by using sitecut rafters onto the hip board and infill ceiling joistsspanning onto the hip girder. The horizontal top chords ofthe hip trusses require lateral bracing back to the hip girder.

Mono truss supported on hip girder truss Bbottom chord by trusshanger and fixed to topchord as Figure 4.05

28

Standard Centres HipThe most common form of construction for a hip end is thestandard centres hip system. This comprises a number ofidentical flat top hip trusses, spaced at the same centre asthe main trusses, and a multiple girder of the same profile

supporting monopitch trusses off the bottom chord (Figures4.05 and 4.06). The flying rafters on the hip and monopitchtrusses are usually supplied full length and cut back on siteto ensure that they meet the hip board.

Mono truss fixed togirder using truss clip

Mono trusses supported atmultiple girder truss ontruss shoes - See Detail B

Use mini hanger to fix ceiling joist togirder

SECTION 4


SECTION 4


31

Girder Based HipThe girder based hip is supported by a Howe girder at theapex. This, in turn, supports flat top trusses spanning fromthe end wall. To reduce the amount of site infill timbering,mono trusses can be used, spanning from the side wallsonto a multiple flat top truss.

Girder hanger fixed tovertical web of girder Bto support multiple D hipgirder

Truss hanger for singleply truss

Mono truss to be supportedon hip girder truss D bottom chord by truss hanger andfixed to top chordas figure 4.05

30

Standard Set Back HipThe standard set back hip is virtually identical to thestandard centres hip, except that the position of the hipgirder can be chosen to avoid obstructions, such aschimneys, or to ensure the girder is not supported on alightweight lintel.

Mono truss to be supportedon hip girder B bottom chord bytruss hanger and fixed to topchord as figure 4.05

SECTION 4


SECTION 4


33

Two Stage Hip SystemFor spans greater than 11 m, the load on the hip girderis excessive and/or the corner infill area is too large. The two girder hip system solves both of these problems.From the framing plan it can be seen that a shallow girderis used to support the monopitch trusses and a deepergirder carries load from the flying rafters, with intermediatetrusses of each profile being used.

Mono truss to besupported on hip girdertruss C bottom chord bytruss hanger and fixed totop chord as Figure 4.05

32

Site Infill HipThe site infill hip is the basic form of hip end construction,consisting of a multiple girder at the apex positionsupporting the hip boards and loose ceiling joists. Site cutrafters span from the wall plate onto the hip board to formthe roof slopes. No trusses are used in the hip end area.This form of construction is limited to a maximum span of 6 metres.

SECTION 4


SECTION 4


Ceiling infill supported ongirder truss by mini hangersSee Figure 4.05

35

Louvred HipThe louvred hip end is made up of the lower part of a hipend, terminated at the ridge with a vertical face. Theconstruction is straightforward, using a girder truss at thevertical face which supports the hip monopitch trusses offthe bottom chord. A vertical web is provided to support thehip board, with corner framing as for a standard hip. The minimum span for the monopitch truss is span/4.

Mono truss to be supported on girder trussB bottom chord by truss hanger andlaterally restrained at the top

34

Generally the set back (see Figure 4.11) is less than aquarter of the main span and a girder is not required; all trusses being spaced at standard centres.

The depth of the hip truss is dictated by the height aboveceiling level of the gable wall. The return slope isconstructed from site cut rafters, spanning from the gablewall up to the hip board. To provide lateral restraint to thetop chords of the hip trusses, it is important to brace themback to the gable wall.

SECTION 4


SECTION 4


Dutch or Barn HipThis form of hip end takes its name from the traditionalDutch barn roof. The gable wall is built up above theceiling line and a truncated hip end formed. The result isan attractive roof line, relatively simple to achieve andfalling between the gable end and hip end in terms ofcost.

The trusses used in the hip section are a flat top hiptruss with flying rafters cut back on site to meet the hipboard.

37

Cranked or Dogleg IntersectionA cranked or dogleg intersection occurs when two roofsmeet at an angle between 90 and 180. Normally theintersecting roofs have the same span and pitch, but somevariation can be accommodated so long as the ridgeheights match.

The framing plan (Figure 4.16) shows the typicalarrangement whereby girders are positioned on theintersection line and at the end of each leg, these beingused to support loose infill. For small spans, girders A andB may be formed using two or three of the standard profiletrusses nailed together. However, for larger spans, and tosimplify erection, all three girders should have vertical websand matching profiles. For further guidance on the choiceof girder and the infill timbers, reference should be made toSection 6. Although detailed for a duopitch roof, monopitchand asymmetric roofs can be treated in a similar manner.

Except in situations where there are several identical doglegturns, using stubbed trusses as a replacement for site infillwould be too costly. Where this is proposed, special fixingswill be required to support the stubbed trusses on thediagonal girders.

Girder hanger fixed to centralweb to support trussed purlin C

Mono truss D to be supportedon trussed purlin bottom chordby truss hanger and fixed totop chord as Figure 4.05

36

Hip CornersA hip corner is formed when two roofs meet at right angles toeach other. Common variations are shown in Figure 4.13.

As for hip ends, these two systems are very similar andonly the former is illustrated in Figure 4.14 for a corner withequal spans and equal pitch.

There are two common framing systems:

(1) Standard Centres Hip Corner.(2) Standard Set Back Hip Corner.

Girder hanger fixed tovertical webs of girder Dto support multiple Aand B trusses

Mono truss to besupported on hip girdertruss B bottom chord bytruss hanger and fixed totop chord as figure 4.05

SECTION 4


SECTION 4


39

Vertical connections are used in the two extreme casesof small span trusses with steep pitches or very large spantrusses. Since these truss types are seldom required, they arenot considered further.

Horizontal connections occur in two forms:

Structural - The two parts are connected structurally to act as one unit. Attic trusses are frequentlyproduced using this method, asdiscussed later.

Non-structural - The units are designed to workindependently and only nominallyconnected. Since one truss sits on top of the other, they are often referred to as'Top Hat' trussed rafters.

tolerances in the timber and the manufacturing processwill make it very difficult to achieve an acceptable roof line andthe truss would not reflect the design assumptions (Figure 4.22).

The structural action of the top hat truss is one wherebythe lower section supports itself plus the loads transmittedfrom the upper or top hat section. The profile will normally betaken from the hip family with a height generally between 2mand 3m. The upper section is often a Queen Post or finkprofile.

To simplify erection and fixing, the top hat section lapsalongside the lower section and bears on a 50mm x 100mmwall plate. These details should not be revised so as to allowthe two sections to be in continuous contact along theadjoining chords. Small allowable

Most truss profiles can be supplied as multipart trusses.Standard modifications are also applicable.

Attic Trussed RaftersThe attic or room-in-the-roof trussed rafter is a simplemeans of providing the structural roof and floor in thesame component (Figure 4.23b). This offers considerableadvantages over loose timber construction:

(1) There are no restrictions on ground floor layout since thetrusses span onto external walls.

(2) Attic trusses are computer designed and factoryassembled units, resulting in increased quality assurance.

(3) Complex, labour intensive site joints are not required.

(4) Attic trusses can be erected quickly, offering costsavings and providing a weathertight shell earlier.

(5) Freedom to plan the first floor room layout.

(6) A complete structure is provided, ready to receive rooffinishes, plasterboard and floorboarding. If we compare an 8m fink truss (Figure 4.23a) with an equivalent 8m attic truss(Figure 4.23b), it can be seen that the chord timbers haveincreased in width and depth.

There are two reasons for this:(1) The attic truss supports approximately 60% more load thana fink truss of the same span and pitch. This difference in loadis made up of plasterboard ceilings and wall construction, fullsuperimposed floor loading and floor boarding.

(2) The lack of triangulation in an attic truss will result inincreased timber sizes. Predominantly 44mm or 47mm thicktimber is used, with depths ranging from 145mm to 245mm.

38

As spans increase and the design of purlins in solid timberbecomes more difficult, it may be necessary to introducetrussed purlins at the ridge to support monopitch trusses(Figure 4.17). This increases the prefabricated area andreduces the site framed area.

Scissor TrussesThe term scissor truss is used to describe a truss with asloping bottom chord. The three recognised variations areshown in Figure 4.18. These trusses are used either toincrease internal headroom without raising the eaves or as a feature in, for example, a church building.

Generally, the difference in pitch between the rafter andbottom chord should not be less than 15. For larger spans,this minimum value may have to be increased. Under load, thestructural action of scissor trusses results in a spread of thesupports. This spread is often limited to a maximum of 12mm,which most buildings can accommodate without detriment tothe finishes. By adopting these parameters, spans of 12mhave been achieved.

The Gang-Nail scissor truss will be provided with a horizontalseat, cut to match the specified bearing width, as shown inFigure 4.19. The inclusion of water tanks, hip ends or roofintersections should be discussed with your Gang-Nailfabricator.

Multipart TrussesIn some cases the dimensions of a truss exceed those thatcan be manufactured, delivered or erected as one unit (seeSection 2). To overcome this problem, it is possible to produceand deliver trusses in two or more parts and erect the requiredprofile on site. Connections can either be in the horizontal or vertical plane.

SECTION 4


SECTION 4


41

An indication of span, pitch and room widths which wouldresult in comfortable designs are given in Figure 4.25. All casesoutside of these should be discussed with your Gang-Nailfabricator.

(1) Dormer windows and stairwell openings are formed byplacing multiple girders each side of the openings (see Figure 4.26) and loose framing in between. Place stairwellsparallel to the trusses and position windows opposite eachother.

Attics - Good PracticeThe application of a few basic principles at the concept stageof a project can result in substantial cost savings bymaximising the use of prefabricated components andminimising loose infill areas.

40

Where only two supports are available for attic trussed rafters,the bottom chord tends to hang off the rafters and the verticalwebs are in tension. A central support adds considerably tothe stiffness of the bottom chord, such that it often props therafter and places the vertical webs in compression.

SECTION 4


SECTION 4


43

(4) It is easier to construct attic roofs with gable endsas opposed to hip ends. Nevertheless, hip ends can be used,although the number of supports available influences the easewith which this can be achieved.

(i) Two Supports. Minimum site framing is achieved bystopping the room at the apex of the hip (Figure 4.29a).Alternatively, a multiple attic truss can be provided at the hipto support site framing spanning onto a normal hip girder.In this case, the room extends into the hip area and dormerwindows can be provided in the end elevation (Figure 4.29b).

(ii) Three supports. Where a central support is provided, therooms can be easily extended into the hip area (Figure 4.30a).It is possible to go beyond but this would involve multiplegirders with framing between them (Figure 4.30b).

42

(2) For T intersections, detail a corridor link between the roomareas. This will reduce the site framing required and also allowthe use of a girder truss in some cases where a loadbearingwall is not provided (Figure 4.27). In the non-preferredarrangement a loadbearing wall is essential.

(3) Make use of loadbearing ground floor walls to add extrasupport to the attic trusses. To be effective they should occurwithin the centre fifth of the span and will have most influencewhen placed near mid-span (Figure 4.28).

SECTION 4


SECTION 4


45

6) Where possible, keep the overall height below thetransportable limit. Local conditions may influence this but,generally 4m is an accepted value. Above this height the unitis made in two parts. Unlike the two-part trusses describedearlier, these units must be structurally connected on site toact as one (Figure 4.32). Details of the required connectionwill be provided by the truss supplier.

Attic Truss ModificationsThe lack of triangulation in attic trusses requires thatmodifications, such as stubbed ends and cantilevers, aretreated with care. The rules given in Section 2 do not apply.

(1) Stubbed Ends. To maintain stability the modifiedattic trussrelies on its outer triangles. If these are removed the truss willcollapse (Figure 4.34). The amount the attic truss can bestubbed depends on span, pitch, room width and local windpressures. No simple rules can be given and each case mustbe treated individually.

(2) Cantilevering. Small cantilevers of less than 600mm canoften be accommodated, but it is prudent to check with theGang-Nail fabricator.

Attic PartitionsAll internal partitions should be constructed using timber studsand plasterboard or some other lightweight partitioningmaterial. Blockwork should not be used. Normally anallowance for the weight of the partitions is included in thedesign of the truss. It is therefore unnecessary to provideadditional strength under walls as is the case with loose floorjoists. To provide support to partitions running parallel totrusses, noggings should be used as shown in Figure 4.35.

(7) Try to locate openings on a 600mm grid to match the trussspacing. This can often reduce the number of trusses required(Figure 4.33).

44

(5) Gable windows are easier to construct and usually cheaperthan dormer windows. Small rooflights can be accommodatedwithin the standard truss spacing.

A typical dormer window and framing details are shown inFigure 4.31. Multiple trusses must be located each side of theopening which, ideally, should not be wider than 1200mm.Larger openings are possible but they require larger infillareas at additional cost.

SECTION 4


SECTION 4


Attic BracingThe principles of bracing are described in detail in Section 7.These apply to attic roofs, although it is worth emphasising thetreatment of the diagonal brace (see Figures 7.09 and 7.10).

47

Tailing joist fixed towall plate with trussclip

Purlin supported on trusshangers

46

Attic ServicesThe lower void area is an ideal place to locate service runs,allowing lateral runs to be positioned between the bottomchords (Figure 4.36). In some instances, access formaintenance is provided into this area via a small fire resistanthatch in the wall.

The structural action of an attic trussed rafter is entirelydifferent to that of a floor joist. The accepted practice ofnotching floor joists is totally unacceptable for an attic trussedrafter. This could easily halve the strength of the member. DO NOT NOTCH OR DRILL ANY MEMBER.

The result, in this case 1.4, must always be rounded up.In our example, therefore 2-ply girders are required.This approach is only valid where the maximum widthsupported by any girder does not exceed three times thestandard truss spacing. Where specific designs for the girderare provided, larger openings can be accommodated. Infillmust be supported at every node point by purlins and binders(Figure 4.38a) or by infill joists located at uniform centres alongthe bottom chord (Figure 4.38b).

Where this cannot be achieved, advice should be obtainedfrom the Trussed Rafter Designer. For example, the stairwell inFigure 4.39 prevents a binder being located at node A, hencethe staircase and floor may impose an unacceptable pointload at X.

Girder Trusses and Site InfillTo minimise manufacturing costs and to avoidconfusion on site, it is common practice to use standardtrusses nailed together to form girders rather thanproduce separate girder designs.The number of trusses required to form the girder isdictated by the width of roof that the girder supports, i.e.

Number of trusses = Width of supported roofto form girder standard truss centres

To illustrate this, consider the following example (Figure 4.37):

= -------600

= 1.4

SECTION 4


SECTION 4


4948

Figures 4.40 and 4.41 show acceptable methods for framingaround windows and stairwells and give recommendedconnections and support details.

Alternative Forms of Attic Construction

Cross Wall ConstructionCross wall construction, as shown in Figure 4.42, isparticularly suited to floor plans with large dormer windows orwhere the layout of the staircase and windows prohibiteconomic use of attic trusses. (see Figure 4.26).

Attic Frame ConstructionThis form of construction is illustrated in Figure 4.43. It isparticularly suitable where a concrete floor slab is provided andlarge room widths are required. It is not recommended formost domestic attic roofs for three main reasons:

(1) The saving in the cost of the frames compared to a full attictruss, is outweighed by increased erection costs.

(2) The design of the frames is dependent upon the supportconditions and the stiffness of the floor joists provided byothers.

(3) The horizontal thrust from the frames must be transmittedinto the floor joists and through the splice joint in the joists.These connections will be structurally significant, since theywould be required to transmit in excess of 6kN.

SECTION 4


SECTION 4


5150

Satisfactory performance from trussed rafters is dependentupon the provision of proper bearings to support and restrainthem without causing damage. All too often this detail isneglected, yet it is neither difficult to understand nor expensiveto provide. This section will assist the specifier in ensuring thatgood practice is followed.

Eaves and Support DetailsFigure 5.01 provides illustrations of a range of standard eavesand support details.

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Should be driven through the holes in the fasteners.

Where a truss is to be supported by a trussed rafter girder, a truss hanger should be used. The hanger legs should bewrapped over the bottom chord of the girder or nailed to theweb members, as shown in Figure 5.03. All nail holes shouldbe used.

In situations where trussed rafters are subjected to wind upliftpositive fixing to the supporting structure is required. The fixingmust mobilise sufficient dead weight to counteract the upliftforce. Figure 5.04 shows two possible methods. The twistedstrap ties the truss directly to the support wall.

The wall plate restraint strap relies on the truss being fixed tothe wall plate using a truss clip. In both cases, the length ofthe strap required depends upon the uplift force and theweight of the supporting structure. Where lightweightstructures are employed, e.g. timber frame construction, itmay be necessary for the restraint to be taken down to thefoundations.

Trussed rafters should be supported only at the designedbearing points. It is advisable, therefore, to erect internal non-loadbearing walls after the roof tiling has been completed. This allows deflection to take place under dead load andreduces the risk of cracks appearing in ceiling finishes.Alternatively, if partitions are of brick or block, the final coursecan be omitted until the tiling has been completed.

Where non-loadbearing partitions are pre-made or siteassembled, they should be an easy fit and must not beforced against the underside of the trussed rafter. (Seealso Section 9 'Site Practice'.)

Multiple Trussed RaftersThe eaves details shown earlier in this section also apply tomultiple trussed rafters, but the extra thickness and loadsassociated with these units frequently necessitate alternativefixing details.

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Truss Fixing DetailsGang-Nail trussed rafters are precision engineered computerdesigned components, manufactured under quality controlledfactory conditions. The same care should be taken on sitewhen fixing the trusses and it is strongly recommended thattruss clips are used to secure the trussed rafters to the wallplates or bearing. (Figure 5.02).

Skew nailing should only be considered where theworkmanship on site is of a sufficiently high standard toensure that the fasteners, joints, timber members and bearingswill not be damaged by careless positioning or overdriving ofthe nails. The minimum fixing at each bearing position shouldconsist of two 4.5mm diameter x 100mm long galvanisedround wire nails, which are skew nailed from each side of thetrussed rafter into the wall plate or bearing. Where nailingthrough the punched metal plate cannot be avoided, the nails

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Medium duty hanger to benailed to face web.Legs mustnot be bent over chord or anyother member

Supporting girder: drill andattach the girder hanger with20mm diameter bolts, boltheads on hanger side (two nailholes are provided for temporarysupport). Roundwashers are tobe used under the bolt headsand 60mm square minimum x5mm thick washers used underthe nuts

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Fixing to Wall PlateTruss clips for multiple units are not available. The preferredfixing method uses framing anchors or heavy duty anglebrackets (Figure 5.05). Where wind uplift must be resisted,twisted straps are also required.

Girder SupportIn general, where a multiple truss is supported by a trussedrafter girder, a metal hanger should be used. This shouldsatisfy the following criteria:

(1) Adequate safe working load.

(2) Sufficient bearing length to support the oncoming truss.

(3) Correct width of hanger to suit the oncoming truss.A hanger which is too wide should never be used with packs,as this will result in flexing of the bearing surface and lead tocracks in the ceiling finishes.

(4) The hanger fixing must be in accordance with themanufacturers' requirements and the TrussedRafter Designer should approve the fixing proposed.

A range of medium and heavy duty girder hangers are shownin Figures 5.06 and 5.07. They provide a range offering safeworking loads up to 40 kN and are suitable for mostapplications where members meet at 90.

The medium duty girder hanger is suitable for supporting twoor three 35mm thick trussed rafters nailed together to form a girder, up to a safe working load of 17kN.

The range of heavy duty girder hangers are suitable for two,three and four ply girders of timber thickness 35mm and47mm.

They provide a range offering safe working loads up to 40kN and are suitable for most applications where membersmeet at 90.

A 25mm horn should be detailed on the oncoming girder toallow for the projecting bolt heads.

Where girders intersect at an angle other than 90, orwhere several members come together, special hangerscan be fabricated. An adequate fixing area must beprovided on the support girder and torsional forces must berestrained. Metalwork is usually finished by hot dippedgalvanising, but a paint finish, to the Client's specification, can be provided.

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In line with the objective of this manual, to provide acomprehensive reference guide to the design of trussed rafterroofs, this section considers a number of special details thatoccur. Fire precautions are reviewed, the required connectionsbetween the walls and roof are given, together with supportdetails for water tanks and services. Particular emphasis isdevoted to the detailing of loose infill areas, including framingto dogleg turns.

Water Tank Location and SupportEven a small domestic cold water storage tank weighs a thirdof a tonne, whilst a large tank, say for a hospital building, can exceed 10 tonnes. The following will enable the BuildingDesigner to make adequate provision for a tank in the majorityof buildings:

Advise the Trussed Rafter Designer of the size andlocation of the tank.

For domestic 230 litre or 300 litre net capacity tanks,specify the tank support timbers in accordance withFigure 6.01, stating whether load is spread overthree or four trusses.

For larger tanks, agree the support points with the TrussedRafter Designer and design the support timber in accordancewith BS5268: Part 2.

Where headroom is limited joist hangers can be used, asFigure 6.02.

Chipboard is not recommended for the tank platform.Plywood or timber is preferred.

Where tanks cannot be located within the web configuration,they are usually supported between multiple trusses (Figure 6.03).

Where extra head height is required, for showers and thelike, the tank must not be supported off the webs or hung fromthe rafters. A framework must be designed following theprinciples of Figure 6.01. A typical detail is given in Figure 6.04,which transfers the load to the node points assumed by thetrussed rafter design.

Figure 6.05 illustrates the alternative locations for the tank inan attic truss. The space is sometimes limited requiring smallertanks to be used in tandemto provide the required capacity.

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Support Provided by MasonryThe design of a wall support or a hanger built into the wall,requires a knowledge of the types of masonry and mortarsemployed, the structural action of the wall and considerablemasonry design experience. Delegating this responsibility tothe Trussed Rafter Designer is therefore both wrong andpotentially dangerous. The design of all masonry fixings mustalways remain the responsibility of the Building Designer.

Where trussed rafters are to be fixed to or supported onmasonry, reactions will be provided by the Trussed RafterDesigner. Where these reactions are too large to suitproprietory masonry hangers, horn supports can be used. At internal support walls care should be taken to ensure thesedo not penetrate the masonry, unless secondary fireprotection is provided at the ends of the timber.

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Support for ServicesAs discussed in Section 3, loads from services fall intotwo categories, i.e. major items, which have to be treatedindividually, and service runs, such as pipework and ducting,for which an allowance is made of a uniformly distributed load.

Major ItemsIf major equipment is to be collected together in a plantroom, support should be provided independent of the roofstructure. If, however, a more discrete layout is adopted, itemssuch as fans, control panels, and air handling units, can readilybe supported by the trussed rafter roof system.

Support on the bottom chord follows identical principles tothose for cold water storage tanks discussed earlier.Support from the top chord requires more care in the design of the hanger fixings and should only be considered wheresupport on the bottom chord is impractical. Nailed fixingsdesigned in accordance with BS5268: Part 2 should normallybe used.

Bolted fixings are only applicable where a high degree of sitesupervision is assured, and the chord size is increased indepth to allow for the loss in strength due to the bolt hole.

Service RunsFigures 6.06 to 6.08 detail a number of ways of supportingservice runs. Normally, a pipe or duct will be supported everythree or four trusses. For multiple runs this can lead tooverloading if all services are supported on the same truss.

A more even distribution of load is achieved by staggering thesupports (Figure 6.06a) or by using timber stools which spreadthe load onto the node points of two trusses (Figure 6.07).

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Truss shoe

Mini hangerMini hanger

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Truss shoe

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Access to ServicesSome buildings, for example hospitals, require a large numberof the services to be located in the roof. In these situations theprovision of an access walkway may be necessary. This canusually be achieved within normal web configurations (Figure6.09a and b). Details, such as Figure 6.09c, should beavoided. The lack of triangulation in this form results in aninefficient trussed rafter, requiring twice the amount of timber.

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Loose timbers will be required to support the tiles and ceilingaround the opening. Typical details for framing around achimney are given in Figure 6.11, but the principles apply toother openings also.

Site InfillIt is impractical and uneconomic to prefabricate some sectionsof a roof. These areas are constructed using loose timbers,often referred to as either site infill, loose timbering, siteframing or stick built. Support is nearly always provided bytrussed rafter girders or multiple trusses. These girderssupport purlins and binders at every node point which, in turn,support the infill rafters and ceiling joists. It is essential that allnodes are used, otherwise the girders can be severelyoverstressed. The procedure is best described by example,the following cases being considered in detail: Small infill areas, e.g. hatch and chimney trimming Cranked or dogleg turns Large infill areas

Responsibility for detailing these areas rests with the BuildingDesigner, who should ensure that the Trussed Rafter Designerhas correctly allowed for the loads on girders generated by the infill timber.

Small Infill AreasEvery effort should be made to accommodate openingswithin the trussed rafter design spacing. Where thiscannot be achieved the spacing of the trusses adjacent to theopening may be increased as shown in Figure 6.10, where:

S is the design spacing of the trussed rafters;

B is the distance between the centres of the trimmingtrussed rafter and the adjacent trussed rafter;

C is the nominal width of the required opening, anddoes not exceed 2 x S.

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Where the two trimming trusses each side of the opening arenominally fixed together with nails at 600mm centres along allmembers, an opening of up to three standard spacings maybe used as in Figure 6.12.

The purlins in this case must be 47mm x 169mm, installedvertically using metal hangers as in Figure 6.13. The bindersand ridge board should be increased to 47mm x 169mm andthe trimmer to 47mm x 120mm.

The above recommendations for chimney openingsassume:

that the purlin,binder and ridge board are C24 or better;

a fink profile truss of 12m maximum span at 600mm centres;

standard loading on the rafter and ceiling tie equivalent toconcrete interlocking tiles with a plasterboard ceiling;

for all cases where the roof pitch is less than 25 the purlinsare installed vertically and supported on 2-ply trusses, asFigures 6.12 and 6.13;

the infill rafter and ceiling joist are detailed by the BuildingDesigner.

depending on the design of the chimney flue and stack,appropriate clearance is allowed between timber and chimney.

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Cranked or dogleg turnsCranked or dogleg turns, as described in Section 4,involve large areas of loose infill (Figure 6.14).

Girder C should be a similar profile to Girder B to ensure thatthe node points line up when locating the purlins and binders.Where for small spans and/or small turn angles the total UDLload on Purlin 1 is not greater than 4.5kN, Girder B can be amultiple of truss A. For larger intersections, where the purlinloads exceed 4.5kN, it is recommended that the profiles

of Girders B and C should be chosen to have vertical webs,similar to Figure 6.15. The vertical web width can then easilybe increased to accommodate the fixing for the purlin. Thenumber of bays is dependent on span and pitch but generally2 metres is a comfortable bay size.

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Wide Eaves SoffitsThe majority of roofs project a small distance beyondthe face of the external walls. This is normally referred to as asoffit and is achieved by providing a rafter overhang asdiscussed in Section 2 and illustrated in Figure 6.17a. Wherelarger soffits are required, they are usually provided by eitherthe appropriate rafter overhang or by cantilevering the truss, as in Figure 6.17b.

Detail 'b' is far easier to achieve than Detail 'a', since itis controlled by the span of the trussed rafter and oftena cantilever of Span/4 can be achieved. Detail 'a' on theother hand is controlled by the rafter size and typicallyfor depths up to 145mm and standard loads, the overhangcan be nine times the rafter depth for C24 and TR26 wherethe spacing of the trussed rafters does not exceed 600mmand the roof pitch is not greater than 35.

With some soffit details it is possible to extend the scopeof Detail 'a' by framing the overhang (Figure 6.17c), details ofwhich must be provided by the Building Designer.

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Large Infill AreasLarge infill areas occur where, in order to house substantialwater storage tanks, for example, an uninterrupted roof void isrequired free of any internal web members. Girders are locatedboth sides of the zone, supporting the infill timbers and also ifrequired the items of plant, as figure 6.16. The details followsimilar principles to those discussed already,except that heavyduty hangers may be required where the binders also supportthe plant loads.

Cos25 o

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Gable LaddersGable Ladders are used at gable ends to provide anoverhanging verge. As shown in Figure 6.19, a gable ladderconsists of two rafter members joined together and supportedby cross noggings built into the brickwork. The gable laddershould be nailed to the last truss only after the gable wall isbuilt up and able to provide support. It should never beallowed to hang unsupported. The last truss should be locatedsuch that the ladder projection (a) is not greater than thesupported distance (b).

Where the gable ladder width exceeds the truss spacing,additional support will be required for the tile battens.Internoggings should be used, staggered to assist nailing(Figure 6.20a). Where the rafter depth is at least 120mm, thealternative shown in Figure 6.20b may be used.

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Cantilevered Hip EndsWhere cantilevered hip ends are planned, the BuildingDesigner should ensure that adequate support can beprovided for the corner infill area. Two possible methods areshown in Figure 6.18. For small cantilevers a structural hipboard is used, propped off the corner of the wall and

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cantilevering out to support eaves beams. For largercantilevers, the hip board is replaced by a diagonal girder. The eaves beams in either case support the loose rafters andadequate eaves depth must be allowed to accommodate the beams.

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If the tiling battens are to be discontinued over a party wall(Figure 6.21c), then lateral restraint must be provided inaddition to that required to transfer longitudinal bracing forces.This should consist of straps (or equivalent), adequatelyprotected against corrosion, with a minimum cross sectionalarea of 50mm2. These straps should be spaced at not morethan 1.5m centres and fixed to two rafter members andnoggings on each side of the party wall by 3.35mm x 50mmlong wire nails.

Extraneous SupportTrussed rafters must only be supported at the positionsassumed in design if overstressing, unsightly roof lines, ordamage to the finishes are to be avoided. Two particularproblem areas are party walls and internal partition

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Gang Nail Trussed Rafter Manual

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